The present invention relates to a method of mass spectrometry and a mass spectrometer.
It is known to use a single quadrupole mass filter or a tandem quadrupole mass filter arrangement to perform targeted quantitative analysis. For example, it is known to perform a Single Ion Recording (“SIR”) experiment wherein a single quadrupole mass filter is set so as to only transmit ions having a mass to charge ratio which corresponds to a target analyte of interest within a time window which corresponds to the expected elution time of the target analyte of interest. If multiple target analytes of interest are monitored for at the same time then the resulting signal recorded by an ion detector for a particular target analyte of interest may be summed for a time interval which is significantly shorter than the chromatographic elution time of a single species of analyte of interest. This is called the dwell time. Repetitively monitoring the detector output during several dwell times allows the profile of the eluting chromatographic peak to be recorded. The signal may then be subsequently integrated in order to quantify the targeted analyte of interest.
In a similar manner, a tandem quadrupole arrangement may be used to monitor for a transition from a selected precursor or parent ion of interest which is then fragmented in a collision cell to form product ions. Particular or selected product ions of interest may then be monitored for each analyte. Such an approach is known as Multiple Reaction Monitoring (“MRM”).
In conventional MRM or SIR experiments using quadrupole mass filters it is common for several target mass to charge ratios to be simultaneously monitored and several different target analytes may elute within a similar time period.
According to a conventional approach analyte ions are monitored for sequentially and repetitively such that the interval between signal acquired for each analyte is sufficient to profile the chromatographic elution profile.
For a quadrupole mass filter operated with resolving DC only one species of ions having a particular mass to charge ratio can be monitored for at any particular time. Accordingly, sequentially switching between multiple mass to charge ratios will result in a reduction in the duty cycle and hence the ultimate sensitivity of the system will also be reduced.
The time window during which a particular analyte ion is monitored may be based upon the chromatographic retention time determined using pure standards of the target analytes during a precalibration procedure. For many LC-MS analyses this window is often relatively large compared to the width of an individual chromatographic peak as it is known that the retention time can change unpredictably for a variety of reasons.
The more transitions monitored for in a particular window then the lower the resulting duty cycle of any specific transition and hence the lower ultimate detection limit of all analytes in the time window.
The reason for using a relatively large time window is partly to simplify method development but also more fundamentally to ensure that shifts in retention time do not result in missed transitions and false negatives. Chromatographic peak shifts can occur because of column aging or the presence of matrix or contaminant co-eluting species which can affect the chemistry of the column or because of pH changes in the matrix. These effects can even reverse the elution order of target analytes.
Even though the exact retention time of each component within a retention time window is not deterministic it is likely that some of the analytes will be chromatographically resolved within a given time window. This means that within a retention time window some analyte transitions will continue to be monitored even after they have eluted from the column.
Reiko Kiyonami et al. “Increased Selectivity, Analytical Precision, and Throughput in Targeted Proteomics”, Molecular & Cellular Proteomics, vol. 10, no. 2, 1 Feb. 2011, ISSN: 1535-9476 discloses a method of selected reaction monitoring wherein a first set of primary transitions are continuously monitored during a predefined elution time window. A set of six to eight transitions is acquired in a data-dependent event, triggered when all of the primary transitions exceed a preset threshold.
US 2007/0114374 (Prest) discloses a method for dynamically adjusting the time period of ion detection. In one arrangement, the dwell time may be dynamically altered during the course of experimental acquisition according to properties of the detected signal (e.g. strength or variability).
US 2009/0236513 (Lock) discloses a method of mass spectrometry wherein a designated “trigger ion” is filtered for, fragmented, and then a designated “trigger ion fragment” is scanned for. Upon detection of the designated trigger ion fragment, at least one “confirmatory ion fragment” is scanned for.
It is desired to provide an improved method of mass spectrometry and an improved mass spectrometer.